The present invention relates to gnomonics or sciatherics, the art of constructing solar chonometers. More particularly, it relates to the construction of solar instruments which, in addition to the time of day, can indicate the day of the year.
Solar chomometers are among the most ancient instruments to which man has applied his scientific ability. One of the oldest references to solar chonometers, or sun dials, in biblical, found at Isaiah 38:8. Perhaps the oldest sun dial of known construction is credited to Berossus (c. 300 B.C.) and employed a hollow hemisphere provided with a bead that cast a shadow onto its concave inside surface.
The variety of sun dials that have been developed, including horizontal, vertical, equatorial, armillary, spherical, cross and star dials, has expanded greatly over the centuries. The various types of dials and their principles of operation are described and explained in Sundials, Frank W. Cousins, John Baker Publishers Limited, 1972.
Solar chonometers have, of course, improved considerably over the centuries. A principal improvement is found in the construction of dials that indicate mean solar time (the time indicated by a conventional clock running at a uniform rate) rather than apparent solar time. These two times differ because the length of a true solar day varies from one day to the next due to the ellipticity of the earth's orbit. Complex variations in the difference between apparent solar time and mean solar time are described by the "equation of time", found in tabular form in the widely used Whitacker's Almanac.
There are only four times during the course of a year when the correction required by the equation of time is zero. The discrepancy reaches a maximum of sixteen minutes, nineteen seconds on the third of November.
The most common technique for causing a sun dial to indicate mean solar time is the use of an analemmic style, the style being a fixed member that casts a shadow upon a scale of hours. An analemmic style, or analemma, is curved so that a shadow is cast at different positions on the scale depending upon the sun's declination. Since declination varies with the day of the year, the curvature can be such that the sunlight falling upon the scale is always positioned at a point that is correct in accordance with the value of the equation of time for that day.
A solar chronometer with an analemma formed by a solid body having the shape of a three-dimensional figure eight was patented in Great Britian in 1892 by Major-General Oliver. This instrument requires the alternate use of two differently shaped analemmas, each appropriate for only one half of the year. Another sun dial, attributed to Richard L. Schmoyer, utilizes a plate with an elongated slot for the style and the style must be rotated to face the sun at each reading.
Another improvement in solar chronometers is represented by Ferguson's Chronometer, on display at the Science Museum in London, which corrects for the equation of time using two abutting tapered rods which cast a shadow that indicates a point rather than a line. This shadow is projected onto a chart having a series of curved hour lines that the shadow moves across to indicate the time of day. The sun's declination causes the shadow to move up and down the lines as the date changes so that the curvature of the lines and the position of the shadow combine to build the equation of time into the reading, thus giving mean solar time. While movement of the shadow across the hour lines of Ferguson's chronometer indicates mean solar time, movement on a perpendicular axis aligned with the celestial pole indicates the day of the year.
It is a principal objective of the present invention to provide an improved and more easily read solar chonometer, one that can accurately indicate both the time of day and the day of the year.